The progressive improvement of electronic systems, such as microcontroller and microprocessor based applications for the control of motors, as well as the availability of improved portable power sources, has made the development of efficient electric motor drives for vehicles, as a viable alternative to combustion engines, a compelling challenge. Electronically controlled pulsed energization of windings of motors offers the prospect of more flexible management of motor characteristics. By control of pulse width, duty cycle, and switched application of a battery source to appropriate stator windings, high functional versatility can be achieved.
The above-identified copending related U.S. patent application of Maslov et al., Ser. No. 09/826,423, identifies and addresses the need for an improved motor amenable to simplified manufacture and capable of efficient and flexible operating characteristics In a vehicle drive environment, it is highly desirable to attain smooth operation over a wide speed range, while maintaining a high torque output capability at minimum power consumption. The copending related U.S. application incorporates electromagnet poles as isolated magnetically permeable structures configured as segments in an annular ring, relatively thin in the radial direction, to provide advantageous effects. The above identified Maslov et al. applications recognize that isolation of the electromagnet segments permits individual concentration of flux in each magnetic core segment, with virtually no flux loss or deleterious transformer interference effects from flux interaction with other core segments as compared with prior art embodiments. Operational advantages can be gained by configuring a single pole pair as an autonomous electromagnet. Magnetic path isolation of the individual pole pair from other pole pairs eliminates a flux transformer effect on an adjacent group when the energization of the pole pair windings is switched.
The above-identified copending U.S. patent application Ser. No. 10/173,610 is directed to a control system for a multiphase motor having these structural features. A control strategy is described therein that compensates for individual phase circuit characteristics and offers a higher degree of precision controllability with each phase control loop closely matched with its corresponding winding and structure. Control parameters are specifically identified with characteristics of each respective stator phase. Successive switched energization of each phase winding is governed by a controller that generates signals in accordance with the parameters associated with the stator phase component for the phase winding energized.
While the motors described in the above-identified applications provide operational advantages, these motors and prior art motors do not exhibit uniformly high efficiency at all speeds within a wide operating speed range, even with non-variable loads. For a fixed motor topology, the available magnetomotive force (MMF) is dependent upon the number of winding turns and energization current. The term “motor topology” is used herein to refer to physical motor characteristics, such as dimensions and magnetic properties of stator cores, the number of coils of stator windings and wire diameter (gauge), etc. The available magnetomotive force dictates a variable, generally inverse, relationship between torque and speed over an operating range. An applied energization current may drive the motor to a nominal operating speed. As the motor accelerates toward that speed, the torque decreases, the current drawn to drive the motor decreases accordingly, and thus efficiency increases to a maximum level. As speed increases beyond the nominal speed, additional driving current is required, thereby sacrificing efficiency thereafter. Thus, efficiency is variable throughout the speed range and approaches a peak at a speed well below maximum speed.
Motors with different topologies obtain peak efficiencies at different speeds, as illustrated in FIG. 1. This figure is a plot of motor efficiency versus operating speed over a wide speed range for motors having different topologies. The topologies represented in this figure differ solely in the number of stator winding turns. Each efficiency curve approaches a peak value as the speed increases from zero to a particular speed and then decreases toward zero efficiency. Curve A, which represents the motor with the greatest number of winding turns, exhibits the steepest slope to reach peak efficiency at the earliest speed V2. Beyond this speed, however, the curve exhibits a similarly steep negative slope. Thus, the operating range for this motor is limited. The speed range window at which this motor operates at or above an acceptable level of efficiency, indicated as X% in FIG. 1, is relatively narrow.
Curves B through E represent motors with successively fewer winding turns. As the number of winding turns decreases, the motor operating speed for maximum efficiency increases. Curve B attains peak efficiency at speed V3, Curve C at V4, Curve D at V5 and Curve E at V6. Each motor has peak efficiency at a different motor operating speed, and none has acceptable efficiency over the entire range of motor operating speeds.
In motor applications in which the motor is to be driven over a wide speed range, such as in a vehicle drive environment, FIG. 1 indicates that there is no ideal single motor topology that will provide uniformly high operating efficiency over the entire speed range. For example, at speeds above V6 curves A and B indicate zero efficiency. At the lower end of the speed range, for example up to V2, curves C through E indicate significantly lower efficiency than curves A and B.
For motor vehicle drives, operation efficiency is particularly important as it is desirable to extend battery life and thus the time period beyond which it becomes necessary to recharge or replace an on-board battery. The need thus exists for motors that can operate with more uniformly high efficiency over a wider speed range than those presently in use. This need is addressed in the above identified Maslov et al. Application ('030). The approach taken therein is to change, on a dynamic basis, the number of active coils of each stator winding for each of a plurality of mutually exclusive speed ranges between startup and a maximum speed at which a motor can be expected to operate. The speed ranges are identified in a manner similar to that illustrated in FIG. 1 and a different number of the motor stator winding coils that are to be energized are designated for each speed range to obtain maximum efficiency for each of a plurality of operating speed ranges. The number of energized coils are changed when the speed crosses a threshold between adjacent speed ranges. Each winding comprises a plurality of individual, serially connected, coil sets separated by tap connections. Each respective tap is connected by a switch to a source of energization during a single corresponding speed range. The windings thus have a different number of energized coils for each speed range.
Another approach is described in the above-identified copending Gladkov Applications ('053 and '058). Each stator phase winding is configured with a topology different from the topology of each of the other phase windings. Winding topology is characterized by the total number of coil turns in each phase winding and the wire gauge of the coils in each phase winding. Each phase winding differs from each of the other phase windings by the total number of coil turns or by wire gauge, preferably in both respects. With the gauge sizes and total number of coil turns of the phase windings being in inverse relationship with respect to each other, all of the phase windings are provided with substantially the same total coil mass. Phase winding energization can be tailored to obtain maximum efficiency in each of several operating speed ranges from startup to the maximum speed at which a motor can be expected to operate. For a machine structure that accommodates a large number of phases, it is necessary to predefine, for each speed range, which phase windings are to have no voltage applied as well as to identify what predefined voltage magnitude is to be applied to each of the remaining phase windings. The number, and identity, of the phase windings that are to be energized, as well as the magnitude of the individually applied predefined voltages, may differ for each speed range. The predefined optimal voltages should be applied on a dynamic basis in accordance with the sensed speed of the motor. While the predefined voltages for the phase windings can be derived to provide optimal efficiency over the entire motor operating speed range for a given torque, many motor applications exist which require control for variable motor speed, such as in motor vehicles. Motor output torque should be adjusted in accordance with a user's input command that is related to desired speed.
There continues to be a need for optimizing efficiency throughout the operating speed range and for simplifying control of phase winding voltages at variable speed and torque in accordance with user command.